A red giant is a luminous giant star of low or intermediate mass (roughly 0.5–10 solar masses) that is in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower. The appearance of the red giant is from yellow orange to red, including the spectral types K and M, but also class S stars and most carbon stars.

The most common red giants are the so-called red giant branch stars (RGB stars) whose shells are still fusing hydrogen into helium, while the core is inactive helium. Another case of red giants are the asymptotic giant branch stars (AGB) that produces carbon from helium by the triple-alpha process. To the AGB stars belong the carbon stars of type C-N and late C-R.

Prominent bright red giants in the night sky include Aldebaran (Alpha Tauri), Arcturus (Alpha Bootis), and Gamma Crucis (Gacrux), while the even larger Antares (Alpha Scorpii) and Betelgeuse (Alpha Orionis) are red supergiants.

Characteristics

Red giants are stars with radii tens to hundreds of times larger than that of the Sun which have exhausted the supply of hydrogen in their cores and switched to fusing hydrogen in a shell outside the core. The main sequence stars of spectral types A through K are believed to become red giants.

In fact, such stars are not big red spheres with sharp limbs (when one is close to it) as displayed on many images. Due to the very low density such stars may not have a sharp photosphere but a star body which gradually transfers into a 'corona'.

Stellar Evolution

Red giants are evolved from main sequence stars with masses in the range from about 0.5 solar masses to somewhere between 4 and 6 solar masses. When a star initially forms from a collapsing molecular cloud in the interstellar medium, it contains primarily hydrogen and helium, with trace amounts of "metals" (elements with atomic number > 2, i. e. every element except hydrogen and helium). These elements are all equally mixed throughout the star. The star reaches the main sequence when the core reaches a temperature high enough to begin fusing hydrogen (a few million Kelvin) and establish hydrostatic equilibrium. Over its main sequence life, the star slowly converts the hydrogen in the core into helium; its main sequence life ends when nearly all the hydrogen in the core has been used. For the Sun, the main sequence lifetime is approximately 10 billion years; the lifetime is shorter for more massive stars and longer for less massive stars.
When the star exhausts the hydrogen fuel in its core, nuclear reactions in the core stop, so the core begins to contract due to its gravity. This heats a shell just outside the core, where hydrogen remains, initiating fusion of hydrogen to helium in the shell. The higher temperatures lead to increasing reaction rates, producing enough energy to increase the star's luminosity by a factor of 1,000–10,000. The outer layers of the star then expand greatly, beginning the red giant phase of the star's life. Due to the expansion of the outer layers of the star, the energy produced in the core of the star is spread over a much larger surface area, resulting in a lower surface temperature and a shift in the star's visible light output towards the red — hence red giant, even though the color usually is orange. At this time, the star is said to be ascending the red giant branch of the Hertzsprung-Russell (H-R) diagram.
The mechanism that ends the collapse of the core and the ascent up the red giant branch depends on the mass of the star. For the Sun and red giants less than 2.57 solar masses, the core will become dense enough that electron degeneracy pressure will prevent it from collapsing further. Once the core is degenerate, it will continue to heat until it reaches a temperature of roughly 10⁸ K, hot enough to begin fusing helium to carbon via the triple-alpha process. Once the degenerate core reaches this temperature, the entire core will begin helium fusion nearly simultaneously in a so-called helium flash. In more massive stars, the collapsing core will reach 10⁸ K before it is dense enough to be degenerate, so helium fusion will begin much more smoothly, with no helium flash. Once the star is fusing helium in its core, it contracts and is no longer considered a red giant. The core helium fusing phase of a star's life is called the horizontal branch in metal-poor stars, so named because these stars lie on a nearly horizontal line in the H-R diagram of many star clusters. Metal-rich helium-fusing stars instead lie on the so-called red clump in the H-R diagram.

In stars massive enough to ignite helium fusion, an analogous process occurs when central helium is exhausted and the star switches to fusing helium in a shell, although with the additional complication that in many cases hydrogen fusion will continue in a shell at lesser depth. This puts stars onto the asymptotic giant branch, a second red giant phase. More massive stars continue to repeat this cycle, fusing heavier elements in successive phases, each lasting more briefly than the previous.

A solar mass star will never fuse carbon. Instead, at the end of the asymptotic giant branch phase, the star will eject its outer layers, forming a planetary nebula with the core of the star exposed, ultimately becoming a white dwarf. The ejection of the planetary nebula finally ends the red giant phase of the star's evolution.

Stars that do not become red giants

Very low mass stars are thought to be fully convective and thus may not accumulate an inert core of helium, and thus may exhaust all of their fuel without ever becoming red giants. Such stars are commonly referred to as red dwarfs. The predicted lifespan of these stars is much larger than the current age of the universe, and hence there are no actual observations of these stars aging.

Very high mass stars instead develop to supergiant stars that wander back and forth horizontally over the HR diagram, at the right end constituting red supergiants. These usually end their life as type II supernovae.

The Sun as a red giant

The size of the current Sun (now in the main sequence) compared to its estimated size during its red giant phase.

The Sun is expected to become a red giant in approximately five billion years. It is calculated that the Sun will become sufficiently large to engulf the current orbits of the solar system's inner planets, up to Earth, and its radius will expand to a minimum of 200 times its current value. The Sun will lose a significant fraction of its mass in the process of becoming a red giant, and there is a chance that Mars and all the outer planets will escape as their resulting orbits will widen. Mercury and most likely Venus will have been swallowed by sun's outer layer at this time. Earth's fate is less clear. Earth could technically achieve a widening of its orbit and could potentially maintain a sufficiently high angular velocity to keep it from becoming engulfed. In order to do so, its orbit needs to increase to between and . However the results of studies announced in 2008 show that due to tidal interaction between sun and Earth, Earth would actually fall back into a lower orbit, and get engulfed and incorporated inside the sun before the sun reaches its largest size, despite the sun losing about 38% of its mass.
Before this happens, Earth's biosphere will have long been destroyed by the Sun's steady increase in brightness as its hydrogen supply dwindles and its core contracts, even before the transition to a Red Giant. After just over 1 billion years, the extra solar energy input will cause Earth's oceans to evaporate and the hydrogen from the water to be lost permanently to space, with total loss of water by 3 billion years. Earth's atmosphere and lithosphere will become like that of Venus. Over another billion years, most of the atmosphere will get lost in space as well; ultimately leaving Earth as a desiccated, dead planet with a surface of molten rock.